Thermally conducting composition and method for producing the same

ABSTRACT

A thermally conducting composition of the present invention includes (A) a cellulose nanofiber, and (B) at least one type of an inorganic powder selected from a metal oxide and a diamond having an average particle diameter of 50 nm or less. A method for producing the thermally conducting composition includes the steps of preparing a dispersion by adding water or a mixed solvent of water and a hydrophilic solvent to (A) a cellulose nanofiber and (B) at least one type of an inorganic powder selected from a metal oxide and a diamond having an average particle diameter of 50 nm or less; and removing the water or the mixed solvent of water and a hydrophilic solvent from the dispersion. The present invention provides a thermally conducting composition that utilizes a cellulose nanofiber and an inorganic powder having the thermal conductivity at a nano-scale size, can improve the thermal conductivity significantly, and further can have properties such as anisotropy and transparency, and a method for producing the same.

TECHNICAL FIELD

The present invention relates to a thermally conducting composition and a method for producing the same. Specifically, the present invention relates to a thermally conducting composition that can control anisotropy, has characteristics such as high transparency and high flexibility, and is useful as a sheet, a film or a coating, or the like. The present invention also relates to a method for producing the thermally conducting composition.

BACKGROUND ART

As thermally conducting materials and thermally radiating materials, grease, a polymer film and a polymer sheet, etc. having a highly thermally conducting fine inorganic particle are important materials and technical fields of the industry. These thermally conducting films or sheets generally contain a large amount of highly thermally conducting material to exhibit high function. Examples of the highly thermally conducting materials include metal silicon, alumina, magnesium oxide, aluminum oxide, boron nitride, aluminum nitride, silicon carbide. However, one of the problems is that inclusion of the large amount of thermally conducting material sacrifices the characteristics of a polymer material such as flexibility and a light weight.

On the other hand, as a thermally conducting material, graphite having high thermal conductivity in a plane direction has been utilized and put into practical use for releasing heat generated from a heat source in a plane direction. However, the graphite has a huge problem owing to its electric conductivity and breakability in that it may cause a short circuit in an electric circuit when broken pieces of the graphite produced during a manufacturing process drop into the electric circuit. To solve the problem, a composition obtained by covering a graphite film with an insulating polymer-based thermally conducting material, etc. have been proposed and are in practical use. However, these methods reduce the thermal conductivity in a plane direction significantly, and do not solve the problems caused by the breakability of the graphite.

In recent years, several attempts have been made to utilize a cellulose nanofiber as a thermally conducting material (Non-Patent Document 1). It is reported that a film composed of the cellulose nanofiber and boron nitride exhibits high thermal conductivity and transparency (Non-Patent Document 2). However, this film is obtained by using a boron nitride particle that is much larger than the cellulose nanofiber, and the thermal conductivity is still insufficient. Some of the present inventors propose efficient production of the cellulose nanofiber (Patent document 1), and a conductive film formed of a water-dispersible polymer in which the cellulose nanofiber is dispersed in a conductive polymer (Patent Document 2). The cellulose nanofiber is electrically insulating. Nevertheless, some of the present inventors suggest that the combination of the cellulose nanofiber and the conductive polymer improves the conductivity of the cellulose nanofiber.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP 2012-051991A -   Patent Document 2: JP 2012-236983 A

Non-Patent Documents

-   Non-Patent Document 1: Polyfile, 2011 September, P. 22-25 -   Non-Patent Document 2: ACS Nano, Vol. 8, P. 3606-3613, 2014

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, the conductivity is still insufficient in the above conventional technologies, and therefore higher conductivity is desired.

The present invention provides a thermally conducting composition that utilizes a cellulose nanofiber and an inorganic powder having the thermal conductivity at a nano-scale size, can improve the thermal conductivity significantly, and further can have properties such as anisotropy and transparency, and a method for producing the same.

Means for Solving Problem

A thermally conducting composition of the present invention includes a cellulose nanofiber and a fine particle. The thermally conducting composition includes (A) a cellulose nanofiber, and (B) at least one type of an inorganic powder selected from a metal oxide and a diamond having an average particle diameter of 50 nm or less.

A method for producing a thermally conducting composition of the present invention includes preparing a dispersion by adding water or a mixed solvent of water and a hydrophilic solvent to (A) a cellulose nanofiber and (B) at least one type of an inorganic powder selected from a metal oxide and a diamond having an average particle diameter of 50 nm or less, and removing the water or the mixed solvent of water and a hydrophilic solvent from the dispersion, thereby providing the thermally conducting composition.

Effects of the Invention

The composition of the present invention includes the cellulose nanofiber of the component (A) and at least one type of the inorganic powder selected from a metal oxide and a diamond having an average particle diameter of 50 nm or less of the component (B), and thus has high thermal conductivity that cannot be achieved by the component (B) alone. That is, even a composition containing only a small amount of fine particles can exhibit high thermal conductivity because the cellulose nanofiber efficiently adsorbs the component (B). Furthermore, a composition may also include a polymer matrix of the component (E), which allows the composition to have more excellent thermal conductivity. Additionally, properties such as anisotropic thermal conductivity and transparency can be imparted to those compositions. Therefore, the compositions can be applied to, e.g., thermally conducting films or sheets, or a coating material formed on a substrate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph of a composition in one embodiment of the present invention observed under a scanning electron microscope (50000×).

FIG. 2 is a photograph of a composition in another embodiment of the present invention observed under a scanning electron microscope (50000×).

DESCRIPTION OF THE INVENTION

The present inventors conducted numerous studies based on the idea that the thermal conductivity may be increased by causing the cellulose nanofiber to adsorb a thermally conducting fine particle. The fiber diameter of the cellulose nanofiber is several tens of nanometers. The present inventors conceived that it is highly possible to obtain a material having unprecedented properties by using a material with a size that is close to the fiber diameter of the cellulose nanofiber, for example, such as at least one type of an inorganic powder selected from a metal oxide and a diamond having an average particle diameter of 50 nm or less of the component (B).

The cellulose nanofiber of the component (A) used in the present invention is obtained by fibrillating a cellulose fiber, and examples of the cellulose fiber include plant-derived fibers, fibers separated from animal fibers, and bacterial cellulose. Of these, the cellulose fiber separated from vegetable fibers is preferable. The cellulose fiber may be purified chemically or physically, and then fibrillated to form the cellulose nanofiber. The cellulose nanofiber thus obtained has an average fiber diameter of preferably 10 to 500 nm, more preferably 10 to 100 nm, and still more preferably 10 to 50 nm. The cellulose nanofiber has an indefinite length and appears to be a highly branched continuous fiber when observed under a potential microscope. Here, “fibrillation” means that fibers in a massive state are to be separated from each other, and in a liquid, “fibrillation” means that fibers are to be dispersed. Even a composition containing only a small amount of thermally conducting fine particle can exhibit high thermal conductivity because the cellulose nanofiber efficiently adsorbs the component (B).

Examples of the method for producing the cellulose nanofiber include a physical production method and a chemical production method. The physical production method is a method for fibrillating the cellulose fiber by performing a physical treatment on a material containing the cellulose fiber. The chemical production method is a method for fibrillating the cellulose fiber by performing the physical treatment after performing a chemical treatment (oxidization etc.) for facilitating the fibrillation on the cellulose material. The physical production method is a method for fibrillation by applying a high shear to a dispersion in which the cellulose material is dispersed in water etc. The means for applying a high shear include a bead mill, a blender type dispersing machine, a high speed rotation homogenizer, a high pressure homogenizer, a high-pressure injection treatment, and an ultrasonic dispersing machine.

Although water is generally used as a dispersion medium of a dispersion, a water-soluble organic solvent or a mixed solvent of water and a water-soluble organic solvent may also be used. An acid or alkali, an ionic or non-ionic dispersing agent or surfactant, an inorganic salt, or the like may be added to the dispersion medium.

The chemical methods for fibrillation include an oxidation treatment. The oxidation treatment is preferably performed using an N-oxyl compound and an oxidizing agent.

In the present invention, the cellulose nanofiber of the component (A) is preferably used in the form of a dispersion in which the cellulose nanofiber is dispersed in water, a water-soluble organic solvent, or a mixed solvent of water and a water-soluble organic solvent. It is preferable that the cellulose nanofiber concentration in the dispersion is in a range from 0.1 to 10 mass %. The cellulose nanofiber concentration below the above range is not preferable because it requires much time for removing the water-soluble solvent when producing the composition of the present invention. The cellulose nanofiber concentration over the above range is not preferable because excessive viscosity can make it difficult to produce the composition of the present invention, and is likely to result in a nonuniform composition.

The component (B) used in the present invention is at least one type of an inorganic powder selected from a metal oxide and a diamond having an average particle diameter of 50 nm or less. The preferred average particle diameter of the component (B) is 2 nm or more and 50 nm or less.

Examples of the metal oxide of the component (B) used in the present invention include aluminum oxide, zinc oxide, zirconium oxide, silicon oxide, iron oxide, and titanium oxide. The metal oxide may be a mixture of a plurality of metals. These metal oxides need to have an average particle diameter of 50 nm or less. If the average particle diameter is larger than this value, the component (B) cannot have a synergistic effect with the component (A).

Among those metal oxides, alumina and silica exhibit particularly high effects and are preferable in the present invention. The alumina having an average particle diameter of 50 nm or less includes α-alumina and γ-alumina, and both can be used effectively. In general, silica having an average particle diameter of 50 nm or less is amorphous silica, and examples of the amorphous silica include dry silica obtained by oxidizing silicon tetrachloride etc. in an oxyhydrogen flame, and wet silica obtained by neutralizing water glass. Regardless of the production method, the silica having an average particle diameter of 50 nm or less may be used in the present invention. It is necessary that the diamond of the component (B) used in the present invention has an average particle diameter of 50 nm or less, and such diamond is generally produced by a detonation method.

The metal oxide or diamond having an average particle diameter of 50 nm or less of the component (B) may be subjected to surface treatment or substitution of a surface functional group. Although such surface-treated or surface functional group-substituted metal oxide and diamond can be used as the component (B) of the present invention, those having hydrophilicity are preferable and those having dispersibility in water are more preferable to have a synergistic effect with the component (A).

Although no particular limitation is imposed on the composition of the component (A) and the component (B), it is preferable that the content of the component (A) is 5 to 98 mass % and that of the component (B) is 2 to 95 mass % to the total mass of the components (A) and (B). The content of the component (B) is more preferably 5 mass % or more and 90 mass % or less, and particularly preferably 10 mass % or more and 80 mass % or less to the total mass of the components (A) and (B). The content of the component (B) less than 2 mass %, or more than 95 mass % hardly exhibits an effect of improving the thermal conductivity. The present invention relates to a composition including the components (A) and (B) as the main components. Here, the “main components” mean components that are indispensable for achieving the effects (thermal conductivity) of the present invention, and the total amount of the components (A) and (B) is 5 mass % or more when the amount of the composition is 100 mass %.

In the present invention, it is preferred to use a reducing agent as a component (C). Examples of the reducing agent include hydrazine and pyrogallol, and other reducing agents may also be used. Although no particular limitation is imposed on the amount of the reducing agent, it is preferred to use 1 to 50 mass % of the reducing agent with respect to the amount of the component (B). The use of the reducing agent is especially preferred when polymers as described below are used.

The composition of the present invention preferably includes an anionic dispersing agent as a component (D). The anionic dispersing agent used as the component (D) preferably includes at least one group selected from the group consisting of a carboxyl group, a sulfo group, a phosphoric acid group, or salts thereof (a carboxylate group, a sulfonate group, or a phosphate group). Specific examples of the anionic dispersing agent include pyrophosphoric acid, polyphosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, metaphosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, hexametaphosphoric acid, polyacrylic acid, polymethacrylic acid, polyitaconic acid, orthosilicic acid, metasilicic acid, phosphoric acid, polymaleic acid copolymer, humic acid, tannic acid, dodecyl sulfuric acid, dodecylbenzenesulphonic acid, polystyrene sulphonic acid, lignin sulphonic acid, a sulfonic acid group bonded polyester and/or salts thereof. In addition, copolymers with maleic acid, fumaric acid, etc. are preferred. The component (D) used in the present invention also includes a polyaniline derivative treated with sulfonic acid etc. A polymer polymerized from an aniline derivative that is synthesized at a monomer stage may be used as the aniline derivative. Although no particular limitation is imposed on the amount of the component (D) used, the amount of the component (D) is preferably about 0.05 to 1 times as much as the amount of the component (A).

In the composition of the present invention, a polymer may be used as a component (E). The use of the component (E) enabled the composition to have the effects of improving flexibility, mechanical strength, transparency, and adhesion to other materials that come into contact with the composition. Examples of the polymer of the component (E) include polyolefin such as polyethylene or polypropylene, chlorinated polyolefin, fluorinated polyolefin, polystyrene, polyester, polyamide, polyacetal, polycarbonate, polyethylene glycol, polyethylene oxide, polyacrylic acid, polyacrylic acid ester, polymethacrylic acid ester, and polyvinyl alcohol. Moreover, the component (E) can be, e.g., an epoxy resin, a urethane resin, an acrylic resin, a silicone resin, and precursors thereof, which are to be cured by heat or ultraviolet irradiation after the applied composition is dried. The curable polymers may finally become resinous or elastomeric polymers.

The component (A) is hydrophilic, and the component (B) is often hydrophilic, and, e.g., a water-soluble polymer or a water-dispersible polyamide, polyethylene glycol, a polyacrylic acid-based copolymer, a polymethacrylic acid-based copolymer, a polyacrylic acid ester-based copolymer, a polymethacrylic acid ester-based copolymer, a polyester-based copolymer, and polyvinyl alcohol are suitable for fine dispersion of the components.

However, the use of the component (E) in the composition of the present invention does not depend on whether it is hydrophobic or hydrophilic. The present invention is characterized in that even a hydrophobic polymer can be used in the composition of the present invention. No limitation is imposed on the amount of the component (E) used as long as the effects of the composition of the present invention are maintained. In other words, the amount of the component (E) is determined according to the intended use of the composition of the present invention.

When the metal oxide of the component (B) has an average particle diameter of 50 nm or less, the component (E) having a refractive index in a range from 1.45 to 1.60 is preferable for improving the transparency of the composition of the present invention.

The components (A) and (B), and optionally the components (C) to (E) are dispersed in a mixed solvent of water and a hydrophilic solvent to form a dispersion. The composition of the present invention can be molded after removing the mixed solvent of water and a hydrophilic solvent from the dispersion. No limitation is imposed on the method of removing the mixed solvent of water and a hydrophilic solvent in the production of the composition of the present invention. Examples of the specific method include volatilization, filtration, and centrifugation.

It is preferable that the molded composition is heated at a temperature in a range from 80° C. to 200° C. to remove the water and the hydrophilic solvent completely. When performing heating, simultaneous heating and pressing can adjust the shape of the molded composition. When the composition includes the component (C), this heat treatment is particularly preferable because it can improve the thermal conductivity of the composition.

When the component (E) is a hydrophobic polymer, a pre-form may be prepared in the above manner from the dispersion in which the components (A) and (B), and optionally the components (C) and (D) are dispersed in the mixed solvent of water and a hydrophilic solvent. Then, the preform may be impregnated with the component (E), thus producing the composition of the present invention. During the impregnation, it is effective to decompress the system. When the component (E) is a thermosetting polymer, a pre-polymer composition may be prepared by mixing the component (E) with a cross-linking agent or a curing catalyst. Then, the preform may be impregnated with the pre-polymer composition. The composition impregnated with the component (E) may be finally molded by a heat treatment or hot press into the composition of the present invention.

The composition of the present invention may include, if necessary, reinforcing fillers, bulking fillers, other thermally conducting fillers, thermally conducting compounds, other thermally conducting fillers, polymers for adjusting physical properties, plasticizers, and additives for improving heat resistance, UV resistance, light resistance, oxidation resistance, and flame retardancy. These materials may be used as long as they do not impair the original purpose of the composition of the present invention.

The thermally conducting composition of the present invention preferably has anisotropic thermal conductivity having a higher thermal conductivity in a plane direction than in a thickness direction. For the anisotropic thermal conductivity, it is preferable that the thermal conductivity in the thickness direction is 0.4 W/m·K or more, and the thermal conductivity in the plane direction is 1.0 W/m·K or more. These properties are effective for thermally conducting films or sheets, a coating material formed on a substrate, or the like.

In the present invention, at least one type of an inorganic powder selected from graphene, graphene oxide, and derivatives thereof may also be added. The addition of this inorganic powder can provide the composition having both thermal conductivity and electrical conductivity. The graphene is preferably a thin-layer graphite, including a single-layer graphite and a cleaved graphite having two or more layers. The graphene oxide is produced by oxidizing the graphite, including a single-layer graphite oxide and a cleaved graphite oxide having two or more layers. The derivatives of such graphene and graphene oxide can be used. The derivatives are commonly obtained by chemically modifying the surface of the graphene and the graphene oxide. The graphene or the graphene oxide is a thin layer having a thickness of several nm. The size of the graphene and the graphene oxide in the surface direction can be measured as an average particle diameter by a light scattering method. Although the graphene, the graphene oxide, and the derivatives thereof used in the present invention may have any average particle diameter, those having an average particle diameter of 2 μm or more and 50 μm or less are preferred to achieve high conductivity.

EXAMPLES

Next, the present invention is further specifically described by way of examples and comparative examples. It should be noted that the present invention is not limited to those examples. In the following examples, % indicates mass %.

<Method for Measuring the Average Particle Diameter>

The average particle diameter of the graphene oxide was measured by a light scattering method using “Zeta-potential& Particle size Analyzer ELS-Z” manufactured by Otsuka Electronics Co., Ltd. The values of the average particle diameter of nano alumina and nanodiamond are those indicated in technical documents provided by the manufacturer.

Examples 1-4

A water-soluble cellulose nanofiber dispersion (trade name “BiNFi-s Wma-10002” (solid content 2%) manufactured by Sugino Machine Limited), nano alumina (trade name “Nano Alumina” (average particle diameter of 10 to 20 nm calculated from the measured values of a BET surface area, provided by the manufacturer) manufactured by TECNAN (NAVARREAN NANOPRODUCTS TECHNOLOGY S.L.)), and polyvinyl alcohol (trade name “PVA-505” manufactured by KURARAY CO., LTD.,) were mixed and diluted with water to have a solid concentration of 5%, which then was subjected to an ultrasonic dispersion treatment for 30 minutes, so that a water-soluble dispersion of a composition was prepared. Subsequently, the water-soluble dispersion was subjected to centrifugal defoaming to prepare a mixed water-soluble dispersion. The composition ratio is shown in Table 1. The mass ratio in the table is expressed in parts of each component by mass per 1 part by mass of polyvinyl alcohol. The mixed water-soluble dispersion was coated on a polyethylene terephthalate (PET) film and was allowed to stand until water was volatilized to solidify the sample, and then the sample was heat treated in an oven at a temperature of 100° C. for 30 minutes, thereby forming a translucent and smooth coating film. On the other hand, the mixed water-soluble dispersion was poured into silicone rubber molds and was allowed to stand until water was volatilized to solidify the sample, and then the sample was heat treated in an oven at a temperature of 100° C. for 30 minutes. The resultant coating was peeled-off from the silicone rubber molds, thereby forming a translucent and smooth film. The thickness of each film is shown in Table 1.

The thermal properties of the surface direction and the thickness direction of the film obtained by using the silicone molds in Example 4 were measured by means of “Thermowave Analyzer TA-3” manufactured by BETHEL Co., Ltd. The thermal conductivity in the thickness direction was 0.6 W/mK and the thermal conductivity in the plane direction was 1.4 W/mK. The results showed that even the film obtained by simply pouring the mixed water-soluble solution into the silicone molds had a high anisotropic thermal conductivity in the plane direction.

TABLE 1 Composition (mass ratio with respect to 1 part by mass of polyvinyl alcohol) Thickness (μm) Cellulose Silicone Nano alumina nanofiber on PET rubber mold Ex. 1 0.1 0.1 25 73 Ex. 2 0.2 0.1 23 82 Ex. 3 0.3 0.1 22 77 Ex. 4 0.4 0.1 22 81

Examples 5-12

10 g of water-soluble cellulose nanofiber dispersion (trade name “BiNFi-s Wma-10002” (solid content 2%) manufactured by Sugino Machine Limited), and nano alumina (trade name “Nano Alumina” (average particle diameter of 10 to 20 nm calculated from the measured values of a BET surface area, provided by the manufacturer) manufactured by TECNAN (NAVARREAN NANOPRODUCTS TECHNOLOGY S.L.)) or water-soluble nanodiamond dispersion (trade name “Andante” (primary particle diameter of 4.2 nm, solid content 5%) manufactured by Carbodeon Ltd Oy.), were mixed with a centrifugal mixer, and 8 g (4 g for Example 8) of methanol was further added and mixed with a centrifugal mixer to prepare a dispersion. The composition is shown in Table 2. The dispersion was poured into the inside of a paper frame having a thickness of 1.7 mm mounted on a PET film. After air-drying at room temperature for 2 hours, the film was dried in an oven at a temperature of 70° C. for 20 minutes and further dried in an oven at a temperature of 100° C. for 10 minutes. The obtained film was cut out to have a width of 10 mm and a length of 10 mm, and then subjected to press molding at a temperature of 120° C. under a pressure of 30 kgf/cm² for 5 minutes, thereby forming smooth films (Examples 5 to 8). The composition of the films is shown in Table 2.

Silicone liquids A and B having a refractive index of 1.57 (trade name “ASP-1120-A/B” manufactured by Shin-Etsu Chemical Co., Ltd.), were mixed, and the films of Examples 5 to 8 were immersed with the silicone liquid in a vacuum for 2 hours, so that a polymer-containing composition film impregnated with a silicone polymer was prepared. The resultant film was heated at 150° C. for 20 minutes to obtain cured silicone-containing compositions (Examples 9 to 12). The thickness of the sheets of Examples 5 to 12 was measured, and also surface electric resistance was measured by applying electrodes to the surface of the sheets. These results are shown in Table 2. Moreover, appearance photographs of Examples 5 to 12 are shown in Table 3.

TABLE 2 Composition ratio Surface (mass ratio) electric Nano Nano Cellulose Impregnation Thickness resistance alumina diamond nanofiber of silicone (μm) (MΩ) Ex. 5 10 — 10 No 40 >100 Ex. 6 20 — 10 No 80 70 Ex. 7 30 — 10 No 70 70 Ex. 8 — 10 10 No 50 50 Ex. 9 10 — 10 Yes — — Ex. 10 20 — 10 Yes — — Ex. 11 30 — 10 Yes — — Ex. 12 — 10 10 Yes — —

The surface electric resistance in Table 2 was measured by applying the electrodes to the sheet surface. As shown in Table 2, Examples 5 to 8 have resistance values greater than 50 MΩ that is at a level sufficient for practical use as an electrically insulating material.

As shown in Table 3, the compositions composed of the nano alumina and the cellulose nanofiber have high transparency that further increased when impregnated with the silicone. Even the compositions composed of the nanodiamond and the cellulose nanofiber were found to be slightly transparent.

Example 13

The same water-soluble cellulose nanofiber dispersion and nano alumina as in Examples 5 to 12 were used. 0.9 g of nano alumina and 75 g of methanol were added to 25 g of water-soluble cellulose nanofiber dispersion and mixed with a centrifugal mixer for 30 seconds to prepare a composition dispersion. 10 g of thus obtained composition dispersion was filtered with a membrane filter having 0.8 μm pore size in a pressure reduced condition to separate a liquefied component. The composition film was peeled off from the membrane filter and then subjected to press molding at a temperature of 80° C. under a pressure of 30 kgf/cm² for 5 minutes, thereby forming a smooth film.

Example 14

Addition curing silicone (trade name “X32-679-2” and “XC32-1679-2” manufactured by Momentive Performance Materials Inc.) were mixed at a mass ratio of 10 to 1 to obtain a mixed silicone liquid. The film obtained in Example 13 was immersed in the mixed silicone liquid and then subjected to vacuum impregnation for 20 minutes. The removed film was sandwiched between PET films and further sandwiched between cardboards, and then subjected to press-curing at a temperature of 70° C. for 20 minutes, thereby forming a silicone-impregnated cellulose nanofiber and nano alumina composition film having smoothness and high transparency.

Example 15

The same water-soluble cellulose nanofiber dispersion and nano alumina as in Example 13 were used. 45 g of water-soluble cellulose nanofiber dispersion, 2.7 g of nano alumina and 135 g of methanol were mixed with a homogenizer (15000 rpm) for 5 minutes so that a composition dispersion was prepared. 10 g of thus obtained composition dispersion was filtered with a membrane filter having 0.8 μm pore size in a pressure reduced condition to separate a liquefied component. The composition film was peeled off from the membrane filter and then sandwiched between cardboards and dried under pressure of 1 kg at a temperature of 100° C. for 15 minutes, thereby forming a smooth film.

Example 16

A silicone-impregnated cellulose nanofiber and nano alumina composition film having smoothness and transparency was obtained in the same way as in Example 14 except that the film obtained in Example 15 was used and the vacuum impregnation was performed for 45 minutes.

Example 17

10 g of the same water-soluble nanodiamond dispersion as in Example 8 and 65 g of methanol were added to 25 g of the same water-soluble cellulose nanofiber dispersion as in Examples 5 to 12, and mixed with a centrifugal mixer for 30 seconds to prepare a composition dispersion. 10 g of thus obtained composition dispersion was filtered with a membrane filter having 0.8 μm pore size in a pressure reduced condition to separate a liquefied component. The composition film was peeled off from the membrane filter and then subjected to press molding at a temperature of 80° C. under a pressure of 30 kgf/cm² for 5 minutes, thereby forming a smooth film.

Example 18

A silicone-impregnated cellulose nanofiber and nanodiamond composition film having smoothness and slight transparency was obtained in the same way as in Example 14 except that the film obtained in Example 17 was used.

Example 19

10 g of the same water-soluble nanodiamond dispersion as in Example 17 and 65 g of water (industrial purified water manufactured by SANEI Co., ltd.) were added to 25 g of the same water-soluble cellulose nanofiber dispersion as in Example 17, and mixed with a centrifugal mixer for 30 seconds to prepare a composition dispersion. 10 g of thus obtained composition dispersion was filtered with a membrane filter having 0.8 μm pore size in a pressure reduced condition to separate a liquefied component. The composition film was peeled off from the membrane filter and then subjected to press molding at a temperature of 70° C. under a pressure of 30 kgf/cm² for 20 minutes, thereby forming a smooth film.

The thermal properties of a plane direction and a thickness direction of the cellulose nanofiber and nanofiller composition films, and the silicone-impregnated cellulose nanofiber and nanofiller composition films obtained in Examples 13 to 19 were measured using “Thermowave Analyzer TA-3” manufactured by BETHEL Co., Ltd. The results are shown in Table 4.

TABLE 4 Composition Thermal conductivity (mass ratio) (W/m · K) Cellulose Nano Nano Impregnation Plane Thickness nanofiber alumina diamond of silicone direction direction Ex. 13 10 10 — No 1.56 0.52 Ex. 14 10 10 — Yes 1.06 0.90 Ex. 15 10 30 — No 1.52 1.04 Ex. 16 10 30 — Yes 2.00 0.80 Ex. 17 10 — 10 No 1.66 0.47 Ex. 18 10 — 10 Yes 1.51 0.50 Ex. 19 10 — 10 No 2.72 0.81

As can be seen in Table 4, the results confirmed that the composition films including the cellulose nanofiber and the nanofiller have a higher thermal conductivity in the plane direction than in the thickness direction, that is, those films have anisotropic thermal conductivity.

Comparative Example 1

The same water-soluble cellulose nanofiber dispersion as in Examples 5 to 12 was used. 75 g of methanol was added to 25 g of water-soluble cellulose nanofiber dispersion and mixed with a centrifugal mixer for 30 seconds to obtain a composition dispersion. 10 g of thus obtained composition dispersion was filtered with a membrane filter having a pore size of 0.8 μm in a pressure reduced condition to separate a liquefied component. The composition film was peeled off from the membrane filter and then subjected to press molding at a temperature of 80° C. under a pressure of 30 kgf/cm² for 5 minutes, thereby forming a smooth film.

Examples 20-23

Examples 20 and 13 are the same composition, Examples 21 and 15 are the same composition, Examples 22 and 17 are the same composition, and Examples 23 and 19 are the same composition.

TABLE 5 Thermal Composition conductivity (mass ratio) Thermal of solid Cellulose Nano Nano conductivity Porpsity component nanofiber alumina diamond (W/m · K) (%) (W/m · K) Comparative 10 — 2.60 28.2 2.54 Ex. 1 Ex. 20 10 10 — 1.56 44.3 3.70 Ex. 21 10 30 — 1.52 45.3 3.70 Ex. 22 10 — 10 1.66 48.9 4.48 Ex. 23 10 — 10 2.72 47.4 7.10

The thermal conductivity of the composition was determined using the Bruggman Formula assuming that there was no porosity.

φ=(λc−λ _(f))/(λ_(m)−λ_(f))×(Δm/λc)^(1/3)  Formula of Bruggman

However, λ_(m): the thermal conductivity of the composition

λ_(f): the thermal conductivity of the air phase

λc: the thermal conductivity of the composite

φ: porosity

In the above description, λ_(m) is the thermal conductivity under the assumption that there was no porosity. This is considered as the thermal conductivity (solid component thermal conductivity) of a low-porosity material that can be approximated by compressing the composite of the present invention under conditions of, for example, a high temperature, high humidity, high pressure, etc.

As can be seen in Table 5, the composition films including the cellulose nanofiber and the nanofiller have a higher solid component thermal conductivity than those including the cellulose nanofiber alone (Comparative Example 1), precisely, although the coexistence of the nanofiller increases the porosity, the thermal conductivity of the low-porosity or no-porosity material can be improved.

Example 24

A water-soluble cellulose nanofiber dispersion (trade name “BiNFi-s Wma-10002” manufactured by Sugino Machine Limited) was diluted with water to have a solid concentration of 0.02%. A water-soluble nanodiamond dispersion (trade name “Andante” manufactured by Carbodeon Ltd Oy.) was diluted with water to have a solid concentration of 0.02%. 0.2 g of thus obtained diluted water-soluble nanodiamond dispersion was added to 1 g of thus obtained diluted water-soluble cellulose nanofiber dispersion, which then was subjected to an ultrasonic dispersion treatment for 30 minutes so that a water-soluble dispersion of the cellulose nanofiber and the nanodiamond was prepared. Subsequently, the water-soluble dispersion was subjected to centrifugal separation to remove precipitates, and a supernatant thereof was dropped on a copper foil tape to be air-dried to obtain a sample. The result of observation of the sample under a scanning electron microscope is shown in FIG. 1 (photograph). The photograph of FIG. 1 was taken at a magnification of 50,000 times and a mark under the center has a size of 200 nm.

From FIG. 1, large numbers of nanodiamond particles having a size of about 5 nm being present on the surface of the cellulose nanofiber were observed. The nanodiamond was considered to be adsorbed on the surface of the cellulose fiber, and it was considered that this adsorption structure enables the compositions containing only a small amount of fine particles to exhibit excellent thermal conductivity.

Example 25

A water-soluble cellulose nanofiber dispersion (trade name “BiNFis Wma-10002” manufactured by Sugino Machine Limited) was diluted with water to have a solid concentration of 0.02%. A water-soluble sulfonated polyaniline dispersion (trade name “aquaPASS” (solid content 5%) manufactured by Mitsubishi Rayon Co., Ltd.), as an anionic dispersing agent was diluted with water to have a solid concentration of 0.02%. A water-soluble nanodiamond dispersion (trade name “Andante” manufactured by Carbodeon Ltd Oy.) was diluted with water to have a solid concentration of 0.02%. 0.2 g of the diluted nanodiamond water-soluble dispersion and 0.25 g of the diluted water-soluble sulfonated polyaniline dispersion were added to 1 g of the diluted water-soluble cellulose nanofiber dispersion, which then was subjected to an ultrasonic dispersion treatment for 30 minutes, so that a water-soluble dispersion of the cellulose nanofiber and the nanodiamond was prepared. Subsequently, the water-soluble dispersion was subjected to centrifugal separation to remove precipitates, and a supernatant thereof was dropped on a copper foil tape to be air-dried to obtain a sample. The result of observation of the sample under a scanning electron microscope is shown in FIG. 2 (photograph). The photograph of FIG. 2 was taken at a magnification of 100,000 times and a mark under the center has a size of 200 nm.

In FIG. 2, almost all nanodiamond particles observed were present on the surface of the cellulose nanofiber, and the nanodiamond observed in FIG. 1 that was not being absorbed in the cellulose nanofiber was hardly found. It is considered that the amount of the nanodiamond adsorbed on the surface of the cellulose nanofiber increased by adding an anionic dispersing agent. This observation suggests that the thermal conductivity of the composition of the present invention is improved by adding an anionic dispersing agent.

INDUSTRIAL APPLICABILITY

The composition of the present invention can be used for various applications such as films, sheets, a coating material formed on a substrate, coatings, inks, or the like. 

1. A thermally conducting composition including a cellulose nanofiber and a fine particle comprising: (A) a cellulose nanofiber; and (B) at least one type of an inorganic powder selected from a metal oxide and a diamond having an average particle diameter of 50 nm or less, wherein the component (A) is configured to adsorb the component (B).
 2. The thermally conducting composition according to claim 1, wherein the metal oxide of the component (B) is alumina and/or silica.
 3. The thermally conducting composition according to claim 1, wherein the metal oxide or the diamond having an average particle diameter of 50 nm or less of the component (B) is hydrophilic.
 4. The thermally conducting composition according to claim 1, wherein a content of the component (A) is 5 to 98 mass % and a content of the component (B) is 2 to 95 mass % when a total mass of the components (A) and (B) is 100 mass %.
 5. The thermally conducting composition according to claim 1, further comprising an anionic dispersing agent as a component (D).
 6. The thermally conducting composition according to claim 1, further comprising a polymer as a component (E).
 7. The thermally conducting composition according to claim 6, wherein the polymer of the component (E) has a refractive index in a range from 1.45 to 1.60.
 8. A method for producing the thermally conducting composition according to claim 1, the method comprising: preparing a dispersion by adding water or a mixed solvent of water and a hydrophilic solvent to (A) a cellulose nanofiber and (B) at least one type of an inorganic powder selected from a metal oxide and a diamond having an average particle diameter of 50 nm or less; and removing the water or the mixed solvent of water and a hydrophilic solvent from the dispersion.
 9. The method for producing the thermally conducting composition according to claim 8, wherein the composition is formed into a predetermined shape by heat treatment at a temperature of 80° C. to 200° C. after removing the water or the mixed solvent of water and a hydrophilic solvent from the dispersion.
 10. The method for producing the thermally conducting composition according to claim 8, wherein the composition is impregnated with the polymer of the component (E) after removing the water or the mixed solvent of water and a hydrophilic solvent from the dispersion.
 11. The method for producing the thermally conducting composition according to claim 10, wherein the composition is impregnated with the component (E), and then the polymer of the component (E) is cured. 